validation error messages and Ajax

[eluser]Ajaxian64[/eluser]
To sump up the question would be:
Do the validation process with our own check function work with ajax call ?

I mean It seems that

Code:

$this->form_validation->set_rules('dateEvent', 'Date', 'required|callback_dateEvent_check');

function dateEvent_check() {
$this->form_validation->set_message('dateEvent', 'Date bla bla bla');
return false;
}

This mechanism doesn't work...
My Json return <p>Unable to access an error message corresponding to your field name.<\/p>